This game is currently unavailable ( Dependency failure ) in Steam

This morning I tried to play some CoD:Black Ops 2 and after the game had updated, I couldn’t start the game. I kept getting an error:

After some poking around, I found out that there were actually other components of the game that were still installing. In my case, it was the Zombies addon. I was able to verify this by going into the Steam client and viewing my Library of games. There, I saw the other components still updating:

Steam apparently wasn’t smart enough to let me know there were still updates taking place, but as soon as they were done, I was back to playing Black Ops 2!